Add cream cheese, watercress, lemon zest, and salt to a food processor. Pulse until ingredients are well blended, but leave some chunks of green leaf for color and texture.
Evenly spread cream cheese mixture on 4 slices of bread; top with remaining bread slices to form 4 sandwiches.
Trim crust off the sandwiches, and cut each sandwich diagonally into quarters to make 4 triangles. May be served immediately or stored, covered, in the refrigerator.